Light emitting devices may include, for example, incandescent bulbs, fluorescent lights, and semiconductor light emitting devices, such as light emitting diodes (“LEDs)”. LEDs may include a series of semiconductor layers that may be epitaxially grown on a substrate such as, for example, a sapphire, silicon, silicon carbide, gallium nitride, or gallium arsenide substrate. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, the electrons will “collide” with corresponding holes and recombine, such that a photon of light may be emitted. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device, where the electron-hole recombination occurs.
LEDs are typically monochromatic light sources that appear to emit light having a single color. Thus, the spectral power distribution of the light emitted by LEDs may be centered about a “peak” wavelength, which is the wavelength where the spectral power distribution or “emission spectrum” of the LED reaches its maximum as detected by a photodetector. The width of the spectral power distribution of LEDs is typically between about 10 nm and 30 nm, where the width may be measured at half of the maximum illumination on each side of the emission spectrum (this width may be referred to as the full width at half maximum or “FWHM” width).
To generate white light, LED lamps have been provided that include several LEDs that each emit light of a different color, where the different-colored light emitted by the LEDs combine to produce a desired intensity and/or color of white light. For example, simultaneously energizing red, green, and blue LEDs may result in a combination of light that may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelengths, and spectral power distributions of the source red, green, and blue LEDs.
Additionally or alternatively, white light may also be produced by surrounding a single LED with one or more luminescent materials, such as phosphors, that absorb some of the light emitted by the LED and responsively emit light of one or more other wavelengths. This process is also referred to herein as ‘converting’ some of the light emitted by the LED to the light of the other color(s). The combination of the light emitted by the single colored LED that is not converted by luminescent materials and the light of other color(s) that are emitted by the luminescent materials may produce light that appears to be white or near white to an observer.
Some white LEDs may use conventional rare earth doped inorganic red and green phosphors. For example, a nitride based red phosphor and a garnet green/yellow phosphor may be used in combination with a blue emitting LED to generate white light output. For instance, a white LED lamp may be formed by coating a gallium nitride based blue LED with a yellow luminescent material, such as a cerium doped yttrium aluminum garnet phosphor, which is commonly referred to as YAG:Ce. The blue LED produces an emission with a peak wavelength of, for example, about 455 nm. Some of blue light emitted by the LED passes between and/or through the YAG:Ce phosphor particles without being down-converted (i.e., converted to light having a longer wavelength), while other of the blue light emitted by the LED is absorbed by the YAG:Ce phosphor, which becomes excited and emits yellow fluorescence with a peak wavelength of about 550 nm (i.e., the blue light is down-converted to yellow light). The combination of blue light and yellow light that is emitted by the coated LED may appear white to an observer. Such light is typically perceived as being cool white in color, as it is primarily comprises light on the lower half (shorter wavelength side) of the visible emission spectrum. To make the emitted white light appear more “warm” and/or exhibit better color rendering properties, red-light emitting luminescent materials such as Eu2+ doped CaAlSiN3-based phosphor particles may be added to the coating, which increases color rendering at the expense of efficiency.